This invention relates to phased array antennas and, in particular, to a planar, phased array antenna that can receive circularly polarized and linearly polarized waves at high gain and wide bandwidth.
As the number of direct broadcast services increases world-wide, so does the need for a low-cost, compact antenna for consumer use. Currently available satellite dishes are too bulky and too expensive for many potential customers to use. A dish antenna is just a large reflector for intercepting the incoming waves and concentrating the waves at a focus where an antenna element is located. Instead of a large reflector and a single active element, the incoming electromagnetic waves can be received by a plurality of active elements and the signals from the elements are additively combined. This is done by spacing the active elements one wavelength (or an integral number of wavelengths) apart in a phased array.
At the frequency typically used by direct broadcast satellites (12 Ghz or Ku band), one wavelength is 25 mm. or about one inch. Thus, a large phased array antenna, e.g. 16.times.16 elements, can occupy a relatively small area, e.g. a square eighteen inches on a side. In general, the more elements, the greater the gain of the antenna, although the gain does not increase linearly with the number of elements.
The signals transmitted by satellites can be linearly polarized (horizontal or vertical) or circutarly polarized (left-hand or right-hand). The particular design of a phased array antenna determines what kind of signals it will receive. For example, a relatively compact, planar, phased array used in Europe receives only right-hand, circularly polarized waves, making it unsuitable for North American and other markets, which are presently serviced by satellites transmitting linearly polarized waves.
Because of the small wavelength, the construction of phased array antennas for receiving microwaves is precise and expensive. Precision is needed because a small error can be a large fraction of a wavelength and affect the performance of the array.
In general, an antenna receiving only one type of polarization will have higher gain than an antenna receiving circular and linear polarization. Since the non-commercial consumer does not want to buy more than one antenna in order to obtain access to several satellites, one is faced with the contradictory requirements of providing a low cost, high gain antenna for receiving circularly and linearly polarized waves.
Several planar, phased array antennas have been proposed in the prior art. U.S. Pat. No. 5,270,721 (Tsukamoto et al.) describes a planar antenna including a ground plate, a plate containing the active elements in a 10.times.10 array and separated from the ground plate by an insulating layer, and an aperture plate separated from the elements by a second insulating layer. Each insulating layer is a foam lattice. The patent also discloses mirror-symmetric and asymmetric orientations of pairs of apertures, and corresponding orientations of pairs of antenna elements. The antenna receives only circularly polarized waves.
U.S. Pat. No. 4,857,938 (Tsukamoto et al.) discloses a planar antenna including an aperture plate having elongated, hexagonal apertures arranged in pairs and rotated ninety degrees relative to each other, and fed signals phase shifted ninety degrees relative to each other. The antenna receives only circularly polarized waves.
U.S. Pat. No. 4,816,835 (Abiko et al.) discloses a stacked radiator antenna in which two supply circuits are superimposed in order to receive both left-hand circularly polarized waves and right-hand circularly polarized waves. The power supply circuits are oriented at ninety degrees relative to each other and are separated by a grounded aperture plate. The grounded aperture plate and a radiator plate have square apertures and the radiator plate includes patch elements within the square apertures. The stack, from top to bottom, includes a radiator plate, a first power supply plate, a grounded aperture plate, a second power supply plate, and a ground conductor plate, all but the latter of which must be carefully aligned.
U.S. Pat. No. 3,587,110 (Woodward) discloses a planar array in which the conductors between pairs of elements taper and then branch to provide impedance matching in the array.
The planar phased arrays of the prior art are expensive to manufacture and do not receive both linearly polarized and circularly polarized waves. In view of the foregoing, it is therefore an object of the invention to provide a planar phased array antenna for receiving both linearly polarized and circularly polarized waves.
Another object of the invention is to provide a planar phased array antenna which is less expensive to manufacture.
A further object of the invention is to provide a planar phased array antenna which is more easily assembled than similar antennas of the prior art.